1. Field of the Invention
The present invention relates to protective circuits with power pack for excitation or de-excitation of superconducting coil systems.
2. Description of the Prior Art
In order to apply a current to a superconducting coil, a power pack means having a current source is connected in series with the coil. The supply current of this power pack is continuously increased for excitation until a desired amperage is obtained, then a current switch connected in parallel with a superconducting coil is closed. The superconducting coil current then flows in a closed loop through the current switch and the power pack is shut off. When the superconducting coil current is to be modified, the power pack is turned on again and the current switch is opened. The superconducting coil amperage can then be brought to a desired level using the power pack.
Given this arrangement, if the current switch is inadvertently opened when the power pack is shut off (for example, due to an unintentional quench this is a transition into the normally conducting condition), the entire current of the superconducting coil suddenly flows through the power pack causing a voltage drop in a direction opposite the supply voltage of the power pack, which could severely damage the power pack. In order to prevent such an occurrence, conventional power packs are partially short-circuited at the output in the off condition by thyristors or by contactors.
This arrangement, however, fails to protect the power pack against errors during excitation or de-excitation, since a short-circuit at the output cannot exist during this time period. During a malfunction (for example, an outage of the power supply or a malfunction in the current source), the power pack can no longer supply the same level of current flowing through the superconducting coil during excitation or deexcitation. Therefore, the inductance of the superconducting coil continues to drive current through the power pack and, depending upon the internal resistance of the power pack, this can lead to a high voltage drop and possible destruction of the power pack.
Generally, superconducting coils are independently protected against over voltages by protective devices connected in parallel, which are situated in a cryostat. Energy stored in the superconducting coil is, however, undesireably disseminated through these protective devices due to their conductivity. Unwanted quenching of the superconducting coil occurs due to the heat generated by the protective devices causing evaporation of the helium used to cool the coil.